Continuous slurry polymerization volatile removal

ABSTRACT

A process/apparatus is disclosed for continuously separating a liquid medium comprising diluent and unreacted monomers from a polymerization effluent comprising diluent, unreacted monomers and polymer solids, comprising a continuous discharge of the polymerization effluent from a slurry reactor through a discharge valve and transfer conduit into a first intermediate pressure flash tank with a conical bottom defined by substantially straight sides inclined at an angle to that of horizontal equal to or greater than the angle of slide of the slurry/polymer solids and an exit seal chamber of such diameter (d) and length (l) as to maintain a desired volume of concentrated polymer solids/slurry in the exit seal chamber such as to form a pressure seal while continuously discharging a plug flow of concentrated polymer solids/slurry bottom product of said first flash tank from the exit seal chamber through a seal chamber exit reducer with inclined sides defined by substantially straight sides inclined at an angle to that of horizontal equal to or greater than the angle of slide of the polymer solids which remain after removal of about 50 to 100% of the inert diluent therefrom to a second flash tank at a lower pressure.

This application is a divisional of U.S. application Ser. No.09/313,818, filed May 18, 1999, which is a continuation-in-part of U.S.application Ser. Nos. 09/080,412 and 09/081,392, both filed May 18,1998, which claim the benefit of U.S. Provisional Application No.60/078,859, filed Mar. 20, 1998.

FIELD OF INVENTION

The present invention relates to an apparatus for continuouslyseparating polymer solids from a liquid medium comprising an inertdiluent and unreacted monomers in a slurry polymerization process. Inparticular, the present invention relates to an apparatus forcontinuously separating polymer solids from a liquid medium, drying thepolymer, and recovering the diluent and unreacted monomers with areduction in compression needed for diluent vapor condensation to liquiddiluent for reuse in the polymerization process. In another aspect, theinvention relates to a method for continuously separating polymer solidsfrom a liquid medium. In particular, the invention relates to a methodfor continuously separating polymer solids from a liquid medium, dryingthe polymer, and recovering the inert diluent and unreacted monomers forreuse in the polymerization process.

BACKGROUND OF THE INVENTION

In many polymerization processes for the production of polymer, apolymerization effluent is formed which is a slurry of particulatepolymer solids suspended in a liquid medium, ordinarily the reactiondiluent and unreacted monomers. A typical example of such processes isdisclosed in Hogan and Bank's U.S. Pat. No. 2,285,721, the disclosure ofwhich is incorporated herein by reference. While the polymerizationprocesses described in the Hogan document employs a catalyst comprisingchromium oxide and a support the present invention is applicable to anyprocess producing an effluent comprising a slurry of particulate polymersolids suspended in a liquid medium comprising a diluent and unreactedmonomer. Such reaction processes include those which have come to beknown in the art as particle form polymerizations.

In most commercial scale operations, it is desirable to separate thepolymer and the liquid medium comprising an inert diluent and unreactedmonomers in such a manner that the liquid medium is not exposed tocontamination so that the liquid medium can be recycled to thepolymerization zone with minimal if any purification. A particularlyfavored technique that has been used heretofore is that disclosed in theScoggin et al, U.S. Pat. No. 3,152,872, more particularly the embodimentillustrated in conjunction with FIG. 2 of that patent. In such processesthe reaction diluent, dissolved monomers, and catalyst are circulated ina loop reactor wherein the pressure of the polymerization reaction isabout 100 to 700 psia. The produced solid polymer is also circulated inthe reactor. A slurry of polymer and the liquid medium is collected inone or more settling legs of the slurry loop reactor from which theslurry is periodically discharged to a flash chamber wherein the mixtureis flashed to a low pressure such as about 20 psia. While the flashingresults in substantially complete removal of the liquid medium from thepolymer, it is necessary to recompress the vaporized polymerizationdiluent (i.e., isobutane) in order to condense the recovered diluent toa liquid form suitable for recycling as liquid diluent to thepolymerization zone. The cost of compression equipment and the utilitiesrequired for its operation often amounts to a significant portion of theexpense involved in producing polymer.

Some polymerization processes distill the liquefied diluent prior torecycling to the reactor. The purpose of distillation is removal ofmonomers and light-end contaminants. The distilled liquid diluent isthen passed through a treater bed to remove catalyst poisons and then onto the reactor. The equipment and utilities costs for distillation andtreatment can be a significant portion of the cost of producing thepolymer.

In a commercial scale operation, it is desirable to liquefy the diluentvapors at minimum cost. One such technique used heretofore is disclosedin Hanson and Sherk's U.S. Pat. No. 4,424,341 in which an intermediatepressure flash step removes a significant portion of the diluent at sucha temperature and at such a pressure that this flashed portion ofdiluent may be liquified by heat exchange instead of by a more costlycompression procedure.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to an apparatus for continuouslyseparating polymer solids from a liquid medium comprising an inertdiluent and unreacted monomers. In another aspect, the invention relatesto an apparatus for continuously separating polymer solids from a liquidmedium, drying the polymer, and recovering the diluent and unreactedmonomers with a reduction in compression needed for diluent vaporcondensation to liquid diluent for reuse in a polymerization process. Inanother aspect, the invention relates to a method for continuouslyseparating polymer solids from a liquid medium. In another aspect, theinvention relates to a method for continuously separating polymer solidsfrom a liquid medium, drying the polymer, and recovering the inertdiluent and unreacted monomers for reuse in a polymerization process.

In accordance with the present invention, there is provided an apparatusfor continuously recovering polymer solids from a polymerizationeffluent comprising a slurry of said polymer solids in a liquid mediumcomprising an inert diluent and unreacted monomers. The apparatuscomprises a discharge valve on a slurry reactor, examples of whichinclude slurry loop reactors and stirred tank slurry reactors, for thecontinuous discharge of a portion of the slurry reactor contents into afirst transfer conduit: a first flash tank having a bottom defined bysubstantially straight sides inclined at an angle to the horizontalequal to or greater than the angle of slide of the slurry/polymersolids; wherein the pressure of the first flash tank and temperature ofthe polymerization effluent are such that from about 50% to about 100%of the liquid medium will be vaporized and the inert diluent componentof said vapor is condensable, without compression, by heat exchange witha fluid having a temperature in the range of about 650° F. to about 135°F.: a first flash tank exit seal chamber, communicating with said firstflash tank, of such a length (l) and diameter (d) as to permit such alevel of concentrated polymer solids/slurry to accumulate and form apressure seal in said first flash tank exit seal chamber: a seal chamberexit reducer providing for a continuous discharge of a plug flow ofconcentrated polymer solids/slurry to a second transfer conduit whichcommunicates the concentrated polymer solids/slurry into a second flashtank wherein the pressure of said second flash tank and temperature ofthe concentrated polymer solids/slurry are such that essentially all ofany remaining inert diluent and/or unreacted monomer will be vaporizedand removed overhead for condensation by compression and heat exchangeand the polymer solids are discharged from the bottom of said secondflash tank for additional processing or storage.

The invention provides also a method for the continuous removal of astream of polymerization effluent from a slurry reactor through adischarge valve; increasing the heat content of the polymerizationeffluent during its transit through said first transfer conduit to atemperature below the fusion point of the polymer while continuouslycommunicating the polymerization effluent to a first flash tank having abottom defined by substantially straight sides inclined at an angle tothe horizontal equal to or greater than the angle of slide of theconcentrated polymer solids/slurry; continuously vaporizing from about50% to about 100% of the liquid medium in said first heated flash tankto yield a concentrated polymer solids/slurry and a vapor stream at sucha temperature and pressure that the inert diluent content of said vaporis condensable, without compression, by heat exchange with a fluidhaving a temperature in the range from about 65° F. to about 135° F.;continuously discharging the concentrated polymer solids/slurry fromsaid first flash tank to a first flash tank exit seal chamber of such alength (l) and diameter (d) that a volume of concentrated polymersolids/slurry is continuously maintained so as to form a pressure sealin said first flash tank exit seal chamber; continuously discharging theconcentrated polymer solids/slurry from said first flash tank sealchamber through a seal chamber exit reducer defined by substantiallystraight sides inclined at an angle to that of horizontal equal to orgreater than the angle of slide of the polymer solids which remain afterremoval of about 50 to 100% of the inert diluent therefrom;communicating a continuous plug flow of concentrated polymersolids/slurry from said first flash tank exit seal chamber through saidseal chamber exit reducer to a second transfer conduit whichcommunicates said continuous plug flow of concentrated polymersolids/slurry to a second flash tank; and continuously vaporizingessentially all of any remaining inert diluent and/or unreacted monomerin a second flash tank operated at a lower pressure than said firstflash tank; condensing the vaporized inert diluent and/or unreactedmonomer from said second flash tank by compression and heat exchange;and continuously discharging the essentially dried polymer slurry fromsaid second flash tank for further processing or storage.

The present invention also relates to an apparatus for capturing ahigher weight percentage of polymer solids from a circulating slurry ina loop reactor than the weight percentage of solids in the circulatingslurry. The apparatus includes a conduit having a first end, whereinsaid first end extends for a distance into the loop reactor. The conduitalso has portions defining an opening wherein said opening is positionedrelative to the direction of the circulating slurry. Desirably, theopening may be facing the direction of flow of the circulating slurry.Additionally, a portion of the conduit may extend outwardly from theloop reactor for discharging, continuously or otherwise the polymersolids from the loop reactor.

The present invention also provides a process for capturing a higherweight percentage of polymer solids from a circulating slurry in a loopreactor than the weight percentage of polymer solids in the circulatingslurry. This process includes the step of extending for a distance intoa the loop reactor a conduit having portions defining an opening whereinsaid opening is extends into the circulating slurry. Additionally, thisprocess may include the step of discharging, continuous or otherwise,the polymer solids from the loop reactor through a portion of theconduit extending outwardly from the loop reactor.

The present invention also provides an apparatus for purging polymersolids from a conduit connected to a loop reactor and in fluidcommunication with the loop reactor. This apparatus includes a sensor, afirst valve in fluid communication with the conduit, a second valvepositioned between a first inert diluent and the conduit, wherein thefirst inert diluent is in fluid communication with the conduit betweenthe loop reactor and the first valve. In response to a signal producedby the sensor, the first valve is closed and the second valve is openedallowing the first inert diluent to enter the conduit in sufficientquantities and under sufficient pressure to purge polymer solids fromthe conduit. This apparatus may further include a third valve positionedbetween a second inert diluent and the conduit, wherein the second inertdiluent is in fluid communication with the conduit between the loopreactor and the first valve. In this way, when the first valve is openand the second valve is closed the third valve is opened allowing thesecond inert diluent to enter the conduit.

The present invention also provides a process for purging polymer solidsfrom a conduit connected to a loop reactor and in fluid communicationwith the loop reactor comprising. This process includes the steps of (i)closing a first valve in response to a first signal from a first sensor,wherein the first valve is connected to and in fluid communication withthe conduit, (2) opening a second valve in response to a second signalfrom a second sensor, wherein the second valve is fluid communicationbetween a first inert diluent and the conduit, and wherein the firstinert diluent is in fluid communication with the conduit between theloop reactor and the first valve, and (3) flowing sufficient quantitiesof the first inert diluent under sufficient pressure into the conduit topurge polymer solids from the conduit. In this process the first andsecond sensors may be a common sensor and the first and second signalmay be a common signal.

The present invention also provides an apparatus for returning fines toa polymerization slurry in a loop reactor. The apparatus includes adischarge valve for discharging a portion of the polymerization slurryfrom the loop reactor into a first transfer conduit. The first transferconduit communicates the polymerization slurry into a first flash tank.The first flash tank converts a portion of the polymerization slurryinto a first fluid, such as a vapor. The first fluid includes a portionof the diluent and the fines from the polymerization slurry. A secondtransfer conduit communicates the first fluid to a first cyclone. Thefirst cyclone converts a portion of the first fluid into a second fluid,such as a vapor. The second fluid includes a portion of the diluent andthe fines. A third transfer conduit communicates the second fluid into aheat exchanger. The heat exchanger converts the second fluid into aliquid comprising the diluent and the fines. A fourth transfer conduitreturns the liquid to the polymerization slurry in the loop reactor.This apparatus may also include a first transfer conduit heater for heatexchange between the first transfer conduit heater and thepolymerization slurry.

The present invention also provides a process for returning fines to apolymerization slurry in a loop reactor. The process includes (i)discharging a portion of the polymerization slurry from the loopreactor, (ii) communicating the discharge polymerization slurry into afirst flash tank, (iii) converting in the flash tank a portion of thepolymerization slurry into a first fluid, the first fluid comprising adiluent and the fines, (iv) communicating the first fluid from the firstflash tank to a first cyclone, (v) converting in the cyclone a portionof the first fluid into a second fluid comprising the diluent and thefines, (vi) communicating the second fluid into a heat exchanger, (vii)converting in the heat exchanger the second fluid into a liquidcomprising the diluent and the fines, and (viii) returning the liquid tothe polymerization slurry in the loop reactor.

The present invention further provides an apparatus and process forproducing polymer from a polymerization slurry in a loop reactoroperating at a space time yield greater than 2.8 lbs/hr-gal. In thisinstance, the polymer is formed in the polymerization slurry whichincludes a liquid medium and solids. The polymerization slurry isdischarged into a first transfer conduit. The polymerization slurry isreferred to as a polymerization effluent upon leaving the loop reactor.The polymerization effluent is heated in the first transfer conduit to atemperature below the fusion temperature of the polymer solids. Theheated polymerization effluent is communicated through said firsttransfer conduit to a first flash tank. In the first flash tank, fromabout 50% to about 100% of the liquid medium is vaporized. The vapor iscondensed by heat exchange. Polymer solids are discharge from the firstflash tank to a second flash tank through a seal chamber of sufficientdimension such as to maintain a volume of polymer solids in the saidseal chamber sufficient to maintain a pressure seal. The polymer solidsare then communicated to a second flash tank. In the second flash tank,the polymer solids are exposed to a pressure reduction from a higherpressure in the first flash tank to a lower pressure in said secondflash. The polymer solids are then discharging from said second flashtank. Additionally, the weight percent of solids in the polymerizationslurry may be greater than 47. The loop reactor may be operated at atotal recirculating pumping head/reactor distance of greater than 0.15ft/ft. The loop reactor may also be operated with a recirculatingpumping head greater than or equal to 200 ft. and have more than eightvertical legs, desirably between 10 and 16 vertical legs, more desirablybetween 10 and 12 vertical legs, most desirably 12 vertical legs. Thevolume of polymerization slurry in the loop reactor may be greater than20,000 gallon.

An object of the present invention is to provide both an apparatus andmethod for the continuous two stage flash drying of the polymer solidsfollowing the continuous removal of the polymerization effluentcomprising polymer solids and liquid medium comprising inert diluent andunreacted monomers from a slurry reactor through a point dischargevalve, a continuous solids level control in the first flash tank exitseal chamber that provides a pressure seal therein which enables saidfirst flash tank to operate under a substantially greater pressure thansaid second flash tank while polymer solids are continuously dischargedthrough the seal chamber exit reducer into the second transfer conduitand further into the second flash tank which eliminates plugging in thefirst flash tank and the continuous liquification of from about 50% toabout 100% of the inert diluent vapor by heat exchange rather thancompression.

Another object of the invention is to eliminate the need for a settlingleg on the slurry reactor and the intermittent high pressure pulse inthe slurry reactor caused by periodic discharging of the contents of thesettling leg. Another object of the present invention is to improvesafety by eliminating the possibility of plugging in a settling leg.

Another object of the invention is to eliminate plugging in equipmentdownstream from the discharge valve. In a settling leg of apolymerization reactor polymerization continues and the heat of reactionfurther heats the liquid medium and a potential exists for some of thepolymer solids to dissolve or to fuse together. As the contents of thesettling leg exit the discharge valve, the pressure drop causes flashingof some of the liquid medium which results in cooling the remainingliquid medium causing the dissolved polymer to precipitate which tendsto plug downstream equipment. The present invention which eliminates theneed for a settling leg also eliminates this potential for downstreamequipment plugging by avoiding the initial dissolution or fusion of thepolymer solids.

Another object of the present invention is to increase the reactorthrough-put by the use of continuous discharge and increased ethyleneconcentrations in the liquid medium, e.g., greater than or equal to 4weight percent at reactor outlet, desirably from 4 weight percent to 8weight percent, still more desirably from 5 weight percent to 7 weightpercent. Settling legs limit ethylene concentrations due to an increasedtendency to plug downstream equipment caused by accelerated reactionwithin the settling leg. A continuous polymerization effluent slurryflow allows ethylene concentrations to be limited only by the ethylenesolubility in the liquid diluent in the reactor, thereby increasing thespecific reaction rate for polymerization and increasing reactorthroughput.

Another object of the present invention is to increase the weightpercent (wt %) of polymer solids in the polymerization slurrycirculating in the polymerization zone in the loop reactor. Desirably,the wt % of polymer solids in the polymerization sum is greater than 45,more desirably, from 45 to 65, still more desirably from 50 to 65, andmost desirably from 55 to 65.

Another object of the present invention is to increase the space timeyield (STY), expressed in terms of pounds per hour-gallon (lbs/hr-gal).Desirably, the STY is greater than 2.6, more desirably from 2.6 to 4.0,and most desirably from 3.3 to 4.0.

Other aspects, objects, and advantages of the present invention will beapparent from the following disclosure and FIGS. 1 and 2.

The claimed apparatus and process provide several advantages over theprior art including: (1) allowing for a continuous processing of thecontents of a slurry reactor from the point of discharge of thepolymerization slurry effluent through a discharge valve; a first flashtank; a seal chamber; a seal chamber exit reducer; and therefrom to asecond flash tank, (2) significantly increasing ethylene concentrationin the loop reactor liquid medium thereby increasing reactorthrough-put, (3) significantly increasing the wt % of polymer solids inthe polymerization slurry, (4) significantly increasing reactor spacetime yield and (5) energy consumption is reduced by reducing the need tocompress and/or distill the reactor vapor-liquid effluent. Recyclingcompressors and other downstream equipment can be reduced in size oreliminated.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 and 2 are a schematic diagram illustrating an apparatus forcontinuously separating polymer solids from diluent and unreactedmonomer in accordance with the present invention.

FIG. 3 is an enlarged, cross sectional view of the discharge conduitwith opening extending a distance into the loop reactor and thecirculating polymerization slurry.

FIG. 4 is a schematic view of a pressure control system.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “polymerization slurry” means substantially atwo phase composition including polymer solids and liquid circulatingwithin the loop reactor. The solids include catalyst and a polymerizedolefin, such as polyethylene. The liquids include an inert diluent, suchas isobutane, with dissolved monomer, comonomer, molecular weightcontrol agents, such as hydrogen, antistatic agents, antifouling agents,scavengers, and other process additives.

As used herein, the term “space time yield” (STY) means the productionrate of polymer per unit of loop reactor volume or polymerization slurryvolume.

As used herein, the term “catalyst productivity” means weight of polymerproduced per weight of catalyst introduced into the loop reactor.

As used herein, the term “polymer residence time” means the averageduration that a polymer particle remains within the loop reactor.

The present invention is applicable to any mixture which comprises aslurry of polymer solids and a liquid medium comprising an inert diluentand unreacted polymerizable monomers including slurries resulting fromolefin polymerization. The olefin monomers generally employed in suchreactions desirably include 1-olefins having from 2 up to 8 carbon atomsper molecule. Typical examples include ethylene, propylene, butene,pentene, hexene and octene. Other examples include vinyl aromaticmonomers, like styrene and alkyl-substituted styrene, generallydistributed monomers such as isobutylene and cyclic olefins, such asnorbornene and vinyl norbornene. Typical diluents employed in such olefmpolymerizations include saturated aliphatic hydrocarbons having 3 to 8,preferably 3 to 4 carbon atoms per molecule, such as propane, isobutane,propylene, n-butane, n-pentane, isopentane, n-bexane, isooctane, and thelike. Of these diluents those of 3 to 4 carbon atoms per molecule arepreferred, and isobutane is most preferred.

The rate of discharge of the polymerization effluent is such as to allowa continuous process stream from the slurry loop reactor from the pointof discharge of the liquified polymerization effluent through a singlepoint discharge valve and also through the first flash tank and theassociated vapor recovery and solids recovery systems. The rate ofdischarge of the polymerization effluent is such as to maintain aconstant pressure in the slurry reactor and to eliminate intermittenthigh pressure pulses associated with a discharge of a portion of thereactor contents that occurs with settling legs on slurry reactors.

The temperature to which the polymerization effluent which is dischargedfrom the reactor is heated during transit to the first flash tank forvaporization is below the fusion temperature of the polymer. This may beaccomplished by appropriate heating of this first transfer conduit. Thequantity of heat to be supplied to the polymerization effluent duringits transit through this first conduit to the first flash tank shouldpreferably be at least equal to that quantity of heat which equals theheat of vaporization of that quantity of inert diluent which is to beflash vaporized in the first flash tank. This then will provide for theconcentrated polymer solids formed in the first flash tank to be passedto the second flash tank to pass thereto at a higher solids temperatureand thus facilitates the removal of residual diluent in the pores ofsuch polymer solids by the operation of the second flash tank. Thatquantity of heat transferred to the polymerization effluent during itstransit through the first transfer conduit to the first flash tank mayeven be greater, provided only that the quantity of heat so transferredwill not cause the polymer solids therein to become heated to such atemperature at which they will tend to fuse or agglomerate one withanother.

The concentrated polymer solids/slurry are discharged from the firstflash tank into a first flash tank exit seal chamber of such a length(l) and diameter (d) so as to provide a volume sufficient to maintain avolume of concentrated polymer solids/slurry sufficient to maintain apressure seal in the exit seal chamber. The concentrated polymersolids/slurry are discharged from the exit seal chamber through an exitseal chamber reducer to a second transfer conduit which communicates theconcentrated polymer solids/slurry as a plug flow to a second flashtank. The exit seal chamber reducer is defined by substantially straightsides inclined at an angle to that of horizontal equal to or greaterthan the angle of slide of the concentrated polymer solids/slurry.

The pressure for the first flash step will vary depending on the natureof the diluent and unreacted monomers and the temperature of thepolymerization effluent. Typically, pressures in the range of from about140 psia to about 315 psia can be employed; more preferably from about200 psia to about 270 psia; and most preferably from about 225 psia toabout 250 psia.

The heat exchanging fluid used to condense the vapor from the firstflash step is at a temperature in the range of from about 65°πF. toabout 150° F. A preferred embodiment uses a heat exchange fluid at atemperature of from about 75° F. to about 140° F. A most preferredembodiment uses a heat exchange fluid at a temperature of from about 85°F. to about 130° F.

A further understanding of the present invention will be provided byreferring to FIG. 1 which illustrates a system comprising an embodimentof the invention.

In the embodiment illustrated in FIG. 1, the polymerization is carriedout in a loop reactor 1. It will be understood that while the loopreactor 1 is illustrated with four vertical legs, the loop reactor 1 maybe equipped with more legs, desirably eight or more legs, desirablebetween 8 and 20, more desirable between 8 and 16, most desirable with12 legs. The polymerization slurry is directionally circulatedthroughout the loop reactor 1 as illustrated by arrows A-D by one ormore pumps, such as axial flow pumps, 2A and 2B. Desirably, the loopreactor 1 is equipped with multiple pumps wherein each pump is dedicatedto an even number of legs, such as for example, four legs, six legs,eight legs, etc. Diluent comonomer and monomer are introduced into theloop reactor 1 from the diluent storage vessel 4, the comonomer storagevessel 41, and the monomer source 42 through their respective treaterbeds 37, 38, and 39 through conduits 5, 4 and 3, respectively, connectedto conduit 6. Catalyst is added to the loop reactor 1 through one ormore catalyst feed systems 7A and 7B. Normally, catalyst is introducedin a hydrocarbon diluent.

Polymerization slurry may be removed from the loop reactor by continuousdischarge through a discharge conduit 8A. It will be understood that theloop reactor 1 may be equipped with one or more discharge conduits 8A.It will be also understood that the discharge conduit(s) 8A may beoperated in a continuous or discontinuous mode, but desirably acontinuous mode. The discharge conduit 8A extends for a distance througha portion of the wall of the loop reactor 1 and into the circulatingpolymerization slurry. By extending for a distance into thepolymerization slurry, the discharge conduit 8A may removepolymerization effluent from the circulating polymerization slurry overan area defined from near or adjacent the inside wall of the loopreactor 1 to a distance extending into the circulating polymerizationslurry. In this way, a higher weight percentage of polymer solids may beformed within the conduit 8A and ultimately removed from the loopreactor 1 than the weight percentage of polymer solids within theotherwise circulating polymerization slurry. A pressure control system(not shown in FIG. 1) operates in concert with the discharge conduit 8A.The discharge conduit 8A and the pressure control system 410 are moreclearly illustrated in FIGS. 3 and 4 and will be discussed in greaterdetail below.

The polymerization effluent passes from the discharge conduit 8A to thedischarge valve 8B to a conduit 9 which is provided with a line heater10 and into the first flash tank 11 which separates vaporized liquidmedium from polymer slurry/solids. Conduit 9 has an indirect heatexchange means such as a flash line heater 10.

Vaporized liquid medium comprising diluent and unreacted monomers exitthe first flash tank 11 via transfer conduit 12 through which it ispassed into a separator, such as a cyclone, illustrated by referencenumber 13 which separates entrained polymer solids from the vapor.Polymer solids separated by the cyclone 13 are passed via conduit 14through a dual valving assembly 14A designed to maintain a pressure sealbelow cyclone 13 to a lower pressure second flash tank 15.

The dual valving assemble 14A includes valves 14B and 14C. The valvingassemble 14A in conjunction with conduit 14 operate to periodicallydischarge polymer solids which have collected in the conduit 14 from thecyclone 13. The valving assembly 14A also maintains the pressuredifferential between the higher pressure environment in the cyclone 13and the lower pressure environment in the second flash tank 15. In theoperation of the valving assembly 14A, valves 14B and 14C aresequentially opened and closed. At the beginning of this sequence, thevalve 14B is open and the valve 14C is closed allowing the polymersolids from the cyclone 13 to collect in the conduit 14. Upon thepassage of time and/or the collection of sufficient polymer solids inthe conduit 14, the valve 14B closes capturing a portion of the highpressure environment from the cyclone 13 in the conduit 14. After thevalve 14B closes, the valve 14C opens and the polymer solids collectedin the conduit 14 are forcibly discharged into the flash tank 15 by thedifferential pressure between the higher pressure environment in conduit14 and the lower pressure environment in the flash tank 15. Afterdischarging the polymer solids from conduit 14 into the flash tank 15,the valve 14C closes. Once the valve 14C closes, the valve 14B is openedat which time polymer solids will again collect in conduit 14 from thecyclone 13. The above sequence is then repeated.

Referring back to the first flash tank 11, the concentrated polymersolids/slurry in the bottom of the first flash tank 11 continuouslysettles by sliding along the straight line bottom surface 16 thereofinto the seal chamber 17 which is illustrated in enlargement FIG. 2. Apolymer solids/slurry level 43 is maintained in the seal chamber 17 toeliminate plugging tendencies in first flash tank 11 and to form apressure seal so that the first flash tank 11 can operate at asubstantially higher pressure than the second flash tank 15. Polymerslurry/solids are continuously discharged from the seal chamber 17 intothe lower pressure second flash tank 15. The length (l), diameter (d),and volume of the seal chamber 17 and the geometry of the seal chamberexit reducer 18 are chosen so as to provide a variable residence timeand provide a continuous plug flow of concentrated polymer solids/slurryto minimize “dead” space and reduce plugging tendencies. The sealchamber 17 length must be sufficient to allow particle (polymer solids)level measurement and control.

Particle level measurement and control may be accomplished by a nuclearlevel indicating system 18D. The nuclear level indicating system 18Dincludes a nuclear radiating source (not shown) and receiver or levelelement 18A in signal communication with a level indicating controller18B. In operation, the level element 18A generates a signal proportionalto the particulate level in the seal chamber 17. This signal is conveyedto the level indicating controller 18B. In response to this signal and apreset value, the level indicating controller 18B sends a signal througha conduit (illustrated by broken line 18C) to a control valve 18E whichselectively controls the discharge of polymer solids into a conduit 19.

Typical residence times of the concentrated polymer solid/slurry in theseal chamber 17 are from 5 seconds to 10 minutes, preferable residencetimes are from 10 seconds to 2 minutes and most preferable residencetimes from 15-45 seconds. The continuous plug flow of concentratedpolymer solids/slurry forms a pressure seal wherein the concentratedpolymer solids/slurry have an l/d ratio inside the seal chamber 17 whichis typically 1.5 to 8, preferable l/d is 2 to 6 and most preferable is2.2 to 3. Typically the seal chamber exit reducer 18 sides are inclined,relative to the horizontal, 60-85 degrees, preferable 65-80 degrees andmost preferable 68-75 degrees. The seal chamber exit reducer 18 geometryis defined by substantially straight sides inclined at an angle to thatof horizontal equal to or greater than the angle of slide of theconcentrated polymer slurry/solids and communicates the concentratedpolymer solid/slurry to a second transfer conduit 19 which communicateswith a feed inlet of flash tank 15. In flash tank 15 substantially allof any remaining inert diluent and unreacted monomer in the concentratedpolymerization effluent is vaporized and taken overhead via conduit 20to a second cyclone 21.

Referring now to the cyclone 13, the major portion of the liquid mediumin the polymerization effluent may be been taken to cyclone 13 as vapor.The vapor after having a portion of the entrained catalyst and polymersolids removed is passed via conduit 22 through a heat exchanger system23A wherein the vapor at a pressure from about 140 psia to about 315psia is condensed by indirect heat exchange with a heat exchange fluidsuch as to eliminate the need for compression. The portion of theentrained catalyst and polymer solids not removed by the cyclone 13 aregenerally smaller in size and may be referred to as “fines”, “polymerfines” and/or “catalyst fines”. These fines generally include unreactedand/or under-reacted catalyst.

The heat exchanger system 23A includes a heat exchanger 23E and atempered water circulating pump 23B connected to the heat exchanger 23Eby conduit 23C. A tempered water temperature control valve 23D isconnected to the heat exchanger 23E and water circulating pump 23B byconduits 23F and 23G, respectively. Cooling water from a cooling watersource (not shown) is conveyed via a cooling water conduit 23H into theconduit 23G between the control valve 23D and the circulating pump 23B.A temperature indicating controller (TIC) 23J is connected between thecontrol valve 23D and the conduit 23C. Between the controller 23J andthe conduit 23C resides a temperature element 23K.

The heat exchanger system 23A operates to control the amount of vaporcondensed in the heat exchanger 23E. This is accomplished by controllingthe flow of cooling water introduced into the conduit 23G from theconduit 23H by exhausting heated water formed in the heat exchanger 23E.The heated water from the heat exchanger 23E is conveyed to the controlvalve 23D via the conduit 23F. The heated water exits the control valve23D via the conduit 23I.

More specifically, cooling water from the conduit 23H entering theconduit 23G mixes with circulating tempered water in the conduit 23G,the mixture thereof enters the pump 23B. The water exiting the pump 23Benters the conduit 23C, a portion of which contacts the temperatureelement 23K, in route to the heat exchanger 23E. The temperature element23K generates an signal proportional to the temperature in conduit 23C.The signal is conveyed to the temperature indicating controller 23J. Inresponse to this signal and a preset temperature value, the temperatureindicating controller 23J sends a signal through a signal conduit(illustrated by the broken line 23L) to the control valve 23D whichselectively controls the volume of heated water exiting the heatexchanger system 24A through the conduit 231.

The condensed liquid medium formed at the heat exchanger 23E includesdiluent, unreacted/under-reacted catalyst, polymer solids and unreactedmonomers. This condensed liquid medium is then passed to an accumulator24B via a conduit 22A.

It is desirable to control the amount of vapor condensed in the heatexchanger 23E and to maintain sufficient vapor pressure in theaccumulator 24B. In this way, a pressure control valve 24A can maintainsufficient back pressure on the accumulator 24B. By maintaining asufficient back pressure on the accumulator 24B, a proper operatingpressure is maintained in the first flash tank 11. The pressure controlvalve 24A is actuated by a pressure indicating controller 24C in concertwith a pressure element 24D. The pressure element 24D is in sensingcommunication with the accumulator 24B. The pressure element 24Dgenerates an signal proportional to the pressure in the accumulator 24B.In response to this signal and a preset pressure value, the pressureindicating controller 24C sends a signal through a signal conduit(illustrated by the broken line 24E) to the control valve 24A whichselectively controls the back pressure on the accumulator 24B.

A pump 25 is provided for conveying the condensed liquid medium from theaccumulator 24B back to the polymerization zone by a conduit 26. In thisway, the unreacted/under-reacted catalyst and polymer solids not removedby the cyclone 13 are returned for further polymerization to the loopreactor 1.

The polymer solids in the lower pressure second flash tank 15 are passedvia a conduit 27 to a conventional dryer 28. The vapor exiting thesecondary cyclone 21, after filtration in a filter unit 29, is passed bya conduit 30 to a compressor 31 and the compressed vapors are passedthrough a conduit 32 to a condenser 33 where vapor is condensed and thecondensate is passed through conduit 34 to storage vessel 35. Thecondensed liquid medium in the storage vessel 35 is typically ventedoverhead for removal of light-end contaminants. The inert diluent can bereturned to the process through a treater bed 37 to remove catalystpoisons or distilled in unit 36 for more complete removal of light-endsand then returned to the process through a treater bed.

Turning now to FIG. 3, a portion of a wall 310 of the loop reactor 1through which the discharge conduit 8A extends is illustrated. Thedischarge conduit 8A may extend into the reactor at various angles.Desirably, the discharge conduit 8A extends into the loop reactor atsubstantially a right angle relative to the wall 310.

The wall 310 includes an inside surface 312 and an outside surface 314.The inside surface 312 supports the circulating polymerization slurryillustrated by directional arrows 318. The discharge conduit 8A has atop 316A, and a continuous side 316B. Portions of the side 316B definean opening 320. The opening 320 has a vertical opening dimensions v1 andv2 defined by walls 320A and 320B of the side 316B. Desirably, the v1dimension is greater than the v2 dimension. The opening 320 hashorizontal opening dimensions h1 and h2 (not shown). The opening 320 maybe formed in any suitable shape, such as rectangular, oval, or acombination thereof. In one embodiment, the opening 320 may beconical-shaped or scooped shaped.

The opening 320 communicates with a channel 322 defined by the insidesurfaces of the top 316A and the side 316B. The channel 322 conveyscaptured polymerization slurry, illustrated by directional arrow 324 tothe discharge valve 8B (not shown).

The opening 320 is sized and positioned relative to the direction ofmovement of the circulating polymerization slurry 318. Desirably, theopening 320 is in a substantially facing position to the direction ofthe circulating polymerization slurry 318. More desirably, the opening320 faces the direction of the circulating slurry 318. In this way, aportion of the polymerization slurry 324 containing polymer solids isremoved from the circulating polymerization slurry 318 over an area fromnear or adjacent the inside wall 312 of the loop reactor 1 to a distanceextending into the circulating polymerization slurry 318. In this way, ahigher weight percentage of polymer solids may be formed within theconduit 8A than the weight percentage of polymer solids within theotherwise circulating polymerization slurry.

This weight percentage increase of polymer solids may depend upon thelocation of the discharge conduit 8A along the loop reactor 1, theinsertion depth of the discharge conduit 8A within the loop reactor, thesize and configuration of the opening 320, the orientation of theopening 320 relative to the direction of the circulating polymerizationslurry, and the weight percentage of polymer solids in the circulatingpolymerization slurry 318. For example, between 1 to 5 weight percentagecalculated increase is observed with a discharge conduit 8A having an vldimension of approximately 5 inches and a h1 dimension of approximately1 inch. The discharge conduit 8A was positioned 10 ft downstream of a 90degree bend in the loop reactor 1 in a portion of the loop reactor wall314 adjacent the ground. The discharge conduit 8A extended approximately5.5 inches into the circulating polymerization slurry stream. Thevelocity of the circulating polymerization slurry was in the range of 28to 34 ft/sec with weight percent of polymer solids in the range of 48 to53.

Turning now to FIG. 4, the pressure control system 410 is illustrated.The pressure control system 410 operates to maintain substantiallyuniform pressure within the loop reactor 1 by controlling the dischargeof polymerization effluent from the loop reactor 1 via the dischargeconduit 8A. The control system 410 also operates to prevent plugging ofthe discharge conduit 8A by polymer solids during pressure fluctuationswithin the loop reactor 1 and/or when the flow of polymerizationeffluent from the discharge conduit 8A to conduit 9 is interruptedand/or stopped.

The pressure control system 410 includes a first inert diluent source412, such as isobutane, and an inert diluent conduit 414 incommunication with a loop reactor conduit 416. The flow of inert diluentthrough the inert diluent conduit 414 to the loop reactor conduit 416 iscontrolled by the control valve 418 in concert with a flow element 420and a flow indicator controller 422. The purpose of metering the flow ofinert diluent from the first inert diluent source 412 to the loopreactor 1 is to prevent plugging of the conduit 416 by polymer solids.In this way, a loop reactor pressure element 441 (discussed below), incommunication with the loop reactor conduit 416, may more accuratelymonitor the pressure in the loop reactor 1.

The pressure control system 410 further includes as second inert diluentsource 424 and a third inert diluent source 426. Inert diluent, such asisobutane, from the second inert diluent source 424 flows into a conduit428 towards a control valve 430 which is in fluid communication with aconduit 432. The control valve 430, in concert with a flow element 431and a flow indicator controller 433, meters the flow of inert diluentfrom the second inert diluent source 424 into conduit 432. The conduit432 is in fluid communication with a conduit 434 and the dischargeconduit 8A, terminating in the discharge conduit 8A at a point betweenthe loop reactor 1 and the discharge valve 8B. The purpose of meteringthe flow of inert diluent from the second inert diluent source 422 intothe conduit 432 is to prevent plugging of the conduit 432 by polymersolids which might otherwise back flow into the conduit 432 from thedischarge conduit 8A. Additionally, the flow of inert diluent from thesecond inert diluent source 422 also prevents plugging of the conduit434 and the control valve 440 by polymer solids which might back flowinto conduit 432 from the discharge conduit 8A.

Inert diluent from the third inert diluent source 426 flows into aconduit 438 towards a control valve 440 which is in fluid communicationwith conduit 434. As will be explained in greater detail below, in theevent of a sufficient pressure fluctuation within the loop reactor 1,the control valve 440 operates to initiate a sufficient flow undersufficient pressure of inert diluent from the third inert diluent source426 to purge and/or discharge polymer solids from the discharge conduit8A into the loop reactor 1. In this instance, generally the flow ofinert diluent from the third inert diluent source 426 into the conduit432 will be greater than the flow of inert diluent from the second inertdiluent source 424 into the conduit 432. For example, the flow of inertdiluent from the second inert diluent source 424 to the dischargeconduit 8A may be in a range of 0.5 to less than 2.0 gallons/min. Theflow of inert diluent from the third inert diluent source 426 to thedischarge conduit 8A may be in a range of 2.0 to 20 gallons/min. Theloop reactor pressure element 441 and a pressure indicating controller442 perform several functions. As previously mentioned, the pressureelement 441 monitors the loop reactor 1 pressure via the conduit 416. Inresponse to this pressure, the loop reactor pressure element 441generates an signal proportional to the pressure in conduit 416. Thissignal is conveyed to the pressure indicating controller 442. Inresponse to this signal and a preset pressure value, the pressureindicating controller 442 sends a signal through a signal conduit(illustrated by the broken line 444) to the discharge valve 8B and thecontrol valve 440.

During normal loop reactor operations, the discharge valve 8B ispositioned to permit the flow of polymerization effluent from thedischarge conduit 8A to conduit 9. At the same time, the control valve440 is closed preventing the flow of inert diluent from the third inertdiluent source 426 to the discharge conduit. When sufficient pressurefluctuations occur and/or when partial depressurization in the loopreactor 1 are detected by the loop reactor pressure element 441, thesignal generated by the pressure indicating controller 442 causes thedischarge valve 8B to close and the control valve 440 to open. Byclosing discharge valve 8B, thus interrupting the discharge from theloop reactor 1, pressure within the loop reactor 1 may be restored. Byopening the control valve 440 and flowing sufficient volumes of inertdiluent from the third inert diluent source 426 into the dischargeconduit 8A under sufficient pressure, polymer solids remaining in thedischarge conduit 8A between the discharge valve 8B and the loop reactorI may be flushed out of and/or purged from the discharge conduit 8A andinto the loop reactor 1. Additionally, by maintaining a sufficient flowof inert diluent, continuous or otherwise, into and/or through thedischarge conduit 8A while the discharge valve 8B is closed, the polymersolids within the loop reactor 1 are prevented from entering and/orsubstantially collecting in the discharge conduit 8A and/or plugging thedischarge conduit 8A. Upon return of normal operations, the controlvalve 440 closes terminating the flow of inert diluent from the thirdinert diluent source 426 and the discharge valve 8B opens to resume theflow of polymerization effluent through the discharge conduit 8A intothe conduit 9.

Having broadly described the present invention it is believed that thesame will become even more apparent by reference to the followingexamples. It will be appreciated that the examples are presented solelyfor the purpose of illustration and should not be construed as limitingthe invention.

EXAMPLES Example 1

A typical ethylene polymerization process can be conducted at atemperature of about 215° F. and a pressure of 565 psia. An example ofsuch a process would result in a polymerization effluent of about 83,000pounds per hour comprising about 45,000 pounds per hour of polyethylenepolymer solids and about 38,000 pounds per hour of isobutane andunreacted monomers. The continuously discharged polymerization effluentis flashed in the first flash tank at a pressure of about 240 psia and atemperature of about 180° F. to remove overhead about 35,000 pounds perhour of diluent and unreacted monomer vapors and entrained particulates.Auxiliary heat to impart an additional quantity of heat to thepolymerization effluent is supplied by appropriate heating means duringthe transit between the discharge valve and the first flash tank. Afterremoval of the fines, the isobutane vapor is condensed, withoutcompression, by heat exchange at a pressure of about 240 psia and atemperature of about 135° F. The polymer slurry/solids discharging fromthe bottom of the first flash tank into the seal chamber form acontinuous plug flow of concentrated polymer slurry/solids, whichprovides a pressure seal, with an l/d ratio of the plug of polymerslurry/solids of 2.5 in an 8′4″ long seal chamber having an l/d ratio of5.5 and with a cone angle of about 680 on the seal chamber exit reducer.The residence time of the continuous plug flow of concentrated polymerslurry/solids is about 16 seconds. The concentrated polymerslurry/solids are continuously discharged from the bottom of the firstflash tank at a temperature of about 180° F. and a pressure of about 240psia through a seal chamber, seal chamber exit reducer, and a secondtransfer conduit into a feed inlet on a second flash tank. The remainingliquid medium in the concentrated polymer slurry/solids communicated tothe second flash tank is flashed at a temperature of about 175° F. andat a pressure of about 25 psia to remove about 4,300 pounds per hour ofisobutane and unreacted monomers which are condensed by compression andheat exchange.

Example 2

A typical ethylene polymerization process can additionally be conductedat a temperature of about 215° F. and a pressure of 565 psia. An exampleof such a process would result in a polymerization effluent of about83,000 pounds per hour comprising about 45,000 pounds per hour ofpolyethylene polymer solids and about 38,000 pounds per hour ofisobutane and unreacted monomers. The continuously dischargedpolymerization effluent is flashed in the first flash tank at a pressureof about 240 psia and a temperature of about 175° F. to remove overheadabout 23,000 pounds per hour of diluent and unreacted monomer vapors andentrained particulates. After removal of the fines, the isobutane vaporis condensed, without compression, by heat exchange at a pressure ofabout 240 psia and a temperature of about 112° F. The polymerslurry/solids discharging from the bottom of the first flash tank intothe seal chamber form a continuous plug flow of concentrated polymerslurry/solids, which provides a pressure seal, with an l/d ratio of theplug of polymer slurry/solids of 2.5 in an 8′4″ long seal chamber withan l/d ratio of 5.5 and with a cone angle of about 68° on the sealchamber exit reducer. The residence time of the continuous plug flow ofconcentrated polymer slurry/solids in the seal chamber is about 16seconds. About 60,000 pounds per hour of concentrated polymerslurry/solids are continuously discharged from the bottom of the firstflash tank at a temperature of about 175° F. and a pressure of about 240psia through a seal chamber, seal chamber exit reducer and a secondtransfer conduit into a feed inlet on a second flash tank. The remainingliquid medium in the concentrated polymer slurry/solids communicated tothe second flash tank is flashed at a temperature of about 125° F. andat a pressure of about 25 psia to remove about 16,000 pounds per hour ofisobutane and unreacted monomer which are condensed by compression andheat exchange.

Example 3

An example of a typical ethylene polymerization process was carried outin an eight leg, 20 inch reactor with settling legs having an overalllength of 833 ft and a volume of 11,500 gallons. The reactor wasequipped with a single flash tank (requiring 100% compression of alldiluent discharged from the reactor), a single 460-480 kilowattcirculating pump having a pump head in the range from 85 ft to 10 ft,producing a circulation rate in the range from 21,000 to 28,000 gallonsper minute (gpm) and operated in a discontinuous discharge mode. Thepolymerization temperature and pressure in the reactor would be betweenabout 215° F. to 218° F. and a pressure of 565 psia.

In the process of example 3, the reactor slurry density is in the rangefrom 0.555 gm/cc to 0.565 gm/cc, a polymer production rate range from28,000 pounds to 31,000 pounds per hour while maintaining a reactorsolids concentration weight percentage in the range from 46 to 48 with apolymer residence time in the range from 0.83 to 0.92 hours. Space timeyield (STY) was in the range from 2.4 to 2.7. Example 3 data and resultsare further illustrated in Table 1.

Example 4

Another example of a typical ethylene polymerization processillustrating high polymer solids loading was carried out in an eightleg, 20 inch reactor having an overall length of 833 ft and a volume of11,500 gallons. The reactor in example 4 was equipped dual flash tanks,single discharge conduit, two circulating pumps in series consuming atotal of between 890 and 920 kilowatts producing a total pumping head inthe range from 190 ft to 240 ft, producing a circulation rate in therange from 23,000 to 30,000 gpm and operated in a continuous dischargemode. The polymerization temperature and pressure in the reactor wouldbe between about 217° F. to 218° F. and a pressure of 565 psia.

In the process of example 4 a polymerization effluent was producedhaving a reactor slurry density in the range from 0.588 to 0.592 gm/cc,a polymer production rate in the range from 38,000 to 42,000 pounds perhour while maintaining a reactor solids concentration weight percentagein the range of 54 to 57 with a polymer residence time in the range of0.68 to 0.79 hours. Space time yield (STY) was in the range of 3.3 to3.7. Example 4 data and results are further illustrated in Table 1.

The continuously discharged polymerization effluent is flashed in thefirst flash tank at a pressure of about 240 psia and a temperature ofabout 175° F. to remove overhead about 16,000 pounds per hour of diluentand unreacted monomer vapors and entrained particulates. After removalof the fines, the isobutane vapor is condensed, without compression, byheat exchange at a pressure of about 240 psia and a temperature of about112° F. The polymer slurry/solids discharging from the bottom of thefirst flash tank into the seal chamber form a continuous plug flow ofconcentrated polymer slurry/solids, which provides a pressure seal, withan I/d ratio of the plug of polymer slurry/solids of 2.5 in an 8′4″ longseal chamber with an l/d ratio of 5.5 and with a cone angle of about 68°on the seal chamber exit reducer. The residence time of the continuousplug flow of concentrated polymer slurry/solids in the seal chamber isabout 16 seconds. Concentrated polymer slurry/solids are continuouslydischarged from the bottom of the first flash tank at a temperature ofabout 175° F. and a pressure of about 240 psia through a seal chamber,seal chamber exit reducer and a second transfer conduit into a feedinlet on a second flash tank. The remaining liquid medium in theconcentrated polymer slurry/solids communicated to the second flash tankis flashed at a temperature of about 125° F. and at a pressure of about25 psia to remove about 16,000 pounds per hour of isobutane andunreacted monomer which are condensed by compression and heat exchange.

TABLE 1 ETHYLENE POLYMERIZATION DATA EXAMPLE 3 EXAMPLE 4 Nominal pump(s)size, inches 20 20 Reactor solids concentration, wt. % 46-48 54-57Polymer production rate, K lbs./hr. 28-31 38-42 Reactor circulation pumppower, KW 460-480 890-920 Circulation pump head, ft. 85-110 190-240Circulation rate, GPM 21,000-28,000 23,000-30,000 Reactor slurrydensity, gm/cc 0.555-0.565 0.588-0.592 Reactor temperature, degrees F215-218 217-218 Ethylene concentration, wt. % 4.0-4.4 5.0-6.0 Hexeneconcentration, wt. % 0.13-0.19 0.13-0.19 Heat transfer coefficient,btu/hr-f-ft 215-225 230-245 Reactor volume, gallons 11,500 11,500Reactor length, ft. 833 833 Circulating pump head per reactor length,ft/ft 0.100-0.132 0.228-0.288 Catalyst productivity, lb/lb 2,700-3,0002,700-3,000 Polymer residence time, hrs. 0.83-0.92 0.68-0.79 Space timeyield, lbs/hr - gal 2.4-2.7 3.3-3.7 Isobutane compressed and recycled, %100 45-60

Discussion

In view of the above description and examples, several observationsrelative to the apparatus and process can be made.

It has been observed that by increasing the head and flow capability ofthe loop reactor circulating pump(s), higher weight percent solids canbe circulated in the reactor. It has also been observed that attainingthe necessary head and flow from one pump is increasingly difficult aspercent solids are increased above 45 weight percent and/or reactorlength is increased. Therefore, the use of two pumps in series allows adoubling of pumping head capability and a resulting percent solidsincrease. Increased weight percent solids in the loop reactor increasescatalyst residence time, which for chrome oxide and Ziegler-Nattacatalysts, increases catalyst productivity. One can choose to takeadvantage of higher percent solids and longer residence time by keepingproduction rate constant at reduced catalyst feed rate and improve thecatalyst yield. Another alternative is to maintain catalyst feed rateconstant and increase the reactor throughput and therefor increase STYat nearly constant catalyst productivity. Higher solids also increasesthe weight percent solids removed from the reactor which reducesisobutane processing cost in recycle equipment. Desirably, the highersolids are removed continuously. Continuous discharge may occur througha single point discharge line.

In a loop reactor, it is not always possible to locate the continuousdischarge line in an optimal location to take advantage of centrifugalforce to increase the weight percent solids and therefore reduce theamount of isobutane entrained with the polymer solids. It has beenobserved that a specifically designed pipe as illustrated in FIG. 3inserted into the loop reactor can increase weight percent solidsremoved from the reactor. This pipe insert will function in any sectionof the loop reactor and in a straight section will increase the weightpercent solids to that equal to that in a location which takes advantageof centrifugal force to concentrate solids.

With the development of high weight percent solids circulationcapability in the loop reactor and two-stage flash, the need toconcentrate solids in the reactor discharge is reduced compared to theconventional loop reactor operations having low solids circulation,single-stage flash, continuous discharge line, and continuous dischargeor otherwise. Therefore, the conventional loop reactor settling legs,which are designed to maximize polymer solids concentration prior todischarge, can be replaced with a continuous discharge line, whichsimplifies the system mechanically, reduces capital cost, improvessafety, reduces maintenance and improves reactor control. Settling legsrequire routine maintenance due to their plugging tendency and can formmaterial which plugs downstream polymer handling equipment. Maximum loopreactor ethylene concentration is limited by settling legs due to thetendency for polymer to grow in the legs at elevated ethyleneconcentrations between discharges and therefore plug the leg. Continuousdischarge eliminates this tendency. Another advantage of continuousdischarge is better response to a sudden drop in reactor pressure, whichcan happen if ethylene flow is quickly reduced. Under this condition,settling legs will stop discharging and can plug with polymer withinminutes A development which would increases efficiency of the two-stageflash system is the continuous flash line heater. The heater wouldvaporize up to 100% of the diluent discharged from the reactor with thepolymer which would allow greater recovery of the diluent by theintermediate pressure condenser. Diluent recovery through the firstflash tank would reduce utility and capital cost. Conventional lowpressure single-stage diluent recovery systems include compression,distillation and treatment which have high capital and operating cost.The flash line heater would increase the temperature of the polymer inthe downstream dryer system and would create the potential for lowervolatile levels in the final product, which would lower variable cost,improves safety and aids attainment of environmental standards.

The first flash tank provides an intermediate pressure flash step whichallows for simple condensation of diluent and return to the reactor. Theflash line heater would be capable of supplying sufficient heat tovaporize up to 100% of the diluent in the first flash tank.

Diluent vapor and unreacted/under reacted catalyst/polymer fines gooverhead from the flash tank to the cyclone. The bulk of the polymergoes out the bottom of the first flash tank through the seal chamber tothe second flash tank.

Connected to the bottom of the first flash tank is the seal chamberwhich provides for a low residence time plug flow area to controlpolymer level and maintain pressure in the first flash tank. The sealchamber is designed to accommodate a range of polymer forms fromconcentrated slurry to dry polymer.

The overhead stream from the first flash tank is received by thecyclone, which removes most of the polymer fines and returns them to thebulk of the polymer flow in the second flash tank through a two valvesystem which allows the fines to accumulate between the valves, thendischarge through the bottom valve while maintaining pressure in thefirst flash system.

The overhead stream from the cyclone contains some unreacted/underreacted catalyst and polymer fines. These particles are carried with thediluent vapor to the condenser, entrained with the liquid diluent aftercondensation, collected in the accumulator and returned to the reactorin the diluent. The condensation and accumulator systems are designedand operated to accommodate fines.

The condenser provides for low variable and capital cost liquefaction ofthe diluent removed from the rector with the polymer via the first flashtank. Conventional single flash tank systems flash the polymerizationeffluent to the just above ambient pressure, which requires compressionto liquefy the diluent prior to recycle to the loop reactor. Anintermediate pressure flash provides for condensation with a commonlyavailable cooling medium, such as Plant cooling water. The condensersystem is flushed with diluent and designed to accommodate a level offines without accumulation or plugging. The condenser is cooled by atempered water system which controls the condensation temperature toachieve the proper vapor pressure in the accumulator to allow efficientpressure control by the pressure control valve on the accumulator vent.The condenser tempered water system is a pump-around loop of coolingwater, the temperature of which is controlled by metering in freshcooling water as needed.

The accumulator receives the condensed diluent and catalyst/polymerfines and pumps the mixture back to the loop reactor based on levelcontrol in the accumulator. The accumulator has a bottom shape designedto accommodate fines. A vent on the accumulator purges the accumulateddiluent of light-ends/non-condensables and controls pressure on thefirst flash system.

The second flash tank, operating just above ambient pressure, receivespolymer from the first flash tank seal chamber. Complete vaporization,if not already accomplished in the first flash tank, will occur in thesecond flash tank. Polymer leaves the bottom of the second flash tank tothe dryer system. The flash-line heater would increase the temperatureof the polymer which allows the dryer system to remove residualvolatiles more efficiently and effectively. The overhead of the secondflash tank will be diluent vapor not recovered in the first flash systemand will be filtered and compressed for return to the loop reactor.

While the present invention has been described and illustrated byreference to particular embodiments, it will be appreciated by those ofordinary skill in the art that the invention lends itself to manydifferent variations not illustrated herein. For these reasons, then,reference should be made solely to the appended claims for purposes ofdetermining the true scope of the present invention.

Although the appendant claims have single appendencies in accordancewith U.S. patent practice, each of the features in any of the appendantclaims can be combined with each of the features of other appendantclaims or the main claim.

We claim:
 1. A process for producing polymer comprising: polymerizing inliquid diluent at least one monomer in a loop reactor to produce aslurry of polymer solids; continuously discharging a portion of theslurry from the loop reactor as polymerization effluent; flashing thepolymerization effluent in a first flash to vaporize from about 50% toabout 100% of the liquid medium to produce concentrated polymer effluentand vaporized liquid; condensing at least a portion of the vaporobtained in the first flash without compression; and operating the loopreactor at a space time yield greater than 2.6 lbs/hr-gal.
 2. Theprocess of claim 1, wherein the loop reactor is operated at a space timeyield greater than 2.8 lbs/hr-gal.
 3. The process of claim 2, furthercomprising discharging from the first flash polymer solids to a secondflash through a seal chamber of sufficient dimension such as to maintaina volume of polymer solids/slurry in the seal chamber sufficient tomaintain a pressure seal.
 4. The process of claim 3, wherein the weightpercent solids in the polymerization slurry in the loop reactor isgreater than
 47. 5. The process of claim 4, wherein the first flash isoperated at from about 140 psia to about 315 psia.
 6. The process ofclaim 5, wherein the concentrated polymer effluent and vaporized liquidare continuously separated.
 7. The process of claim 4, wherein theconcentrated polymer effluent slurry is flashed in a second flash tovaporize liquid.
 8. The process of claim 7, wherein the vapor from thefirst flash is condensed by heat exchange.
 9. The process of claim 7,wherein the polymerization slurry is circulated within the loop reactorby multiple pumps and wherein the reactor volume is greater than 20,000gallons.
 10. The process of claim 9, wherein the loop reactor has morethan eight vertical legs.
 11. The process of claim 9, wherein the loopreactor is operated at a total recirculating pumping head/reactordistance of greater than 0.15 ft/ft.
 12. The process of claim 11,wherein the loop reactor is operated with a total pumping head ofgreater than or equal to 200 ft.
 13. The process of claim 1, furthercomprising heating the polymerization effluent.
 14. The process of claim13, wherein the polymerization effluent is heated to a temperature belowthe fusion temperature of the polymer.
 15. The process of claim 13,wherein the quantity of heat supplied to the polymerization effluent isat least equal to that quantity of heat which equals the heat ofvaporization of the liquid medium which is to be flashed in the firstflash.
 16. The process of claim 1, wherein about 50% to about 100% ofthe vaporized liquid from the first flash is condensed withoutcompression.
 17. A process for producing polymer comprising:polymerizing in liquid diluent at least one monomer in a loop reactor toproduce a slurry of polymer solids; continuously discharging a portionof the slurry from the loop reactor as polymerization effluent; flashingthe polymerization effluent in a first flash to vaporize from about 50%to about 100% of the liquid medium to produce concentrated polymereffluent and vaporized liquid; condensing at least a portion of thevapor obtained in the first flash without compression; operating theloop reactor at a space time yield greater than 2.8 lbs/hr-gal;discharging from the first flash polymer solids to a second flashthrough a seal chamber of sufficient dimension such as to maintain avolume of polymer solids/slurry in the seal chamber sufficient tomaintain a pressure seal; wherein the first flash is operated at fromabout 140 psia to about 315 psia; and wherein the concentrated polymereffluent slurry is flashed in a second flash to vaporize liquid.